Increasing the flow rate of traffic is a desirable goal for city planners. Many intersections that previously had stop-lights have been replaced with roundabouts, which allow traffic to flow more naturally based on where it is building up. For intersections that still have lights, however, significant traffic can build up. Also, when a light turns green, not all drivers will move at the same time. These small delays in movement can result in fewer cars moving through the intersection, compounding the traffic backup. Drivers may be distracted, and not notice a light change, or the drivers may not even be able to see a light (due to a large vehicle acting as an obstruction, for example), and will have to wait until they see traffic moving to determine that a light has changed.